vpc: allow outbound tcp/22

Need to be able to ssh *out* of our hosts to communicate with GitHub for
private repos. Add egress TCP port 22 access to the NATSecurityGroup.

cloudformation_templates/edx-reference-architecture.json

@@ -1330,6 +1330,14 @@
             "FromPort":"443",
             "ToPort":"443",
             "CidrIp":"0.0.0.0/0"
+          },
+          {
+            "IpProtocol":"tcp",
+            "FromPort":"22",
+            "ToPort":"22",
+            "CidrIp":{
+              "Ref":"SSHLocation"
+            }
           }
         ]
       }